The present invention relates generally to optical concentrators and, more particularly, to such a multiple stage optical system especially for creating a high-intensity solar light beam.
The well-known technique of concentrating solar radiation into a small area to generate heat has been in existence since the invention of the lens itself. Recently, optical devices, such as large concave mirrors, fresnel lenses, and elongated concave reflectors, have been used as optical collectors, but these all focus sunlight onto a working surface which is at a fixed distance from the respective collector. In other words, the greatest concentration of optical energy takes place only at the focus, and the working distance is therefore highly limited. If the working surface is located out of the focal plane either toward or away from the collector, the photon intensity on this surface will not be at its maximum and, consequently, neither will be the heat flux.
A major advantage of the present invention is that when aimed at the sun, it emits a laser-like output beam (hereafter beam) of concentrated solar radiation which allows the working distance to vary over a fairly large range, i.e. the concentrated radiation is maintained for a much longer distance compared with conventional means of focusing sunlight. A small mirror could be placed in the path of the beam to aim it in a desired direction.
The beam has a very small amount of divergence, the amount of which is comparable to common lasers. Therefore, it can be used to "cut" or "bore," e.g. by burning or melting action, into materials much thicker than when using conventional methods of solar concentration. The cutting ability of the beam depends upon the material to be cut, the intensity (photon flux) of the beam, and the absorptivity of the working surface.
Another major advantage of the present optical system is that when designed accordingly, it has the capability of receiving light from sources or objects other than the sun while still forming a beam as described above. The explanation for this phenomenon is best given in the detailed description of the embodiment. In most cases, however, beam intensity will not be as great as when the sun is the light source.
Along with the above mentioned uses, the beam can also be used for straight-line alignment provided its intensity is low enough as not to cause surface damage. A light source other than the sun could be used in this respect. The varying of beam intensity shall be discussed in the detailed description of the embodiment. Other solar beam uses pertain to any application requiring a high temperature in a small area, or anywhere else where a parallel, narrow light bundle proves useful.
When using a conventional solar collector such as a lens, mirror, etc., a solar light beam of the nature described in this disclosure cannot be formed by simply placing a collimating lens near the focused solar image. This is due to the fact that the sun's image, regardless of collector focal length, has a finite diameter and is not a perfect point image, the latter being a common misconception. Since the sun's image is not a point, the nearby collimating lens will form a bundle of collimated light which will diverge at a fairly rapid rate and could not properly be considered a solid, parallel beam. Likewise, a solar beam of the nature described in this disclosure cannot be formed by placing a reflectance pinhole mask at a solar image with a collimating lens placed closely thereafter. The reason is that the pinhole will allow only a relatively very small portion of light from the sun's image to pass through. In the latter case the resulting beam will be essentially parallel since a pinhole aperture is used but the intensity will be comparatively low since much of the sun's image is being reflected away by the mask around the pinhole. With the present invention, a much larger amount of the light from the initial solar image will be used to form the beam.
In the optical concentrator disclosed in the above patent application, a single optical stage concentrates input light in an intense output light beam. However, certain inefficiencies may be encountered in such optical concentrator due to light waste from aberations and diffraction, due to beam spread of the output beam when the output pinhole is too large, and the like. For minimum beam spread of the output beam it is desirable that the size of the output pinhole cross-section be minimized, say to 0.001 inch or less, but a practical optical design of the optical elements may not permit formation of a collimated bundle width smaller than 0.060 inch, which will cause a large amount of light to be filtered out of the solar image at the pinhole mask, thus reducing efficiency of the optical concentrator.